A "resource" is defined herein as an element of a communication system which may be used directly or indirectly to support communication traffic. For example, electrical energy stored in a satellite battery, or channel capacity of a radio link are resources under this definition. The traffic carrying capacity of a communications system is limited because a finite quantity of resources exists within any system. For example, traffic demand and equipment power requirements (e.g., power for computers, satellite maneuvering energy, etc.) consume system resources.
In any system with limited resources, management of resource usage is desirable to achieve optimal system performance. Specifically, in a communication system containing satellites or other relatively inaccessible nodes that communicate with ground-based equipment, resource management is critical.
FIG. 1 illustrates a simplified diagram of satellite-based, cellular communications system 10, dispersed over and surrounding a celestial body (e.g., earth) through use of orbiting satellites 12.
Satellites 12 occupy orbits 14 that may be low-earth orbits, medium-earth orbits, geosynchronous orbits, or a combination thereof. Low-earth orbits are generally at an altitude of about 600 km to 2000 km, medium-earth orbits at about 2000 km to 20,000 km and geosynchronous orbits at about 42,165 km, but other altitudes can also be used. In the example shown, communications system 10 uses six polar orbit planes, with each orbit plane holding eleven satellites 12 for a total of sixty-six satellites 12. However, this is not essential and more or fewer satellites, or more or fewer orbit planes or combinations of orbiting and geosynchronous satellites, may be used. For clarity, FIG. 1 illustrates only a few of satellites 12.
Satellites 12 communicate with terrestrial equipment which may be any number of radiocommunication Subscriber Units 26, System Control Segment Ground Communication Stations 24 or Gateway Ground Communication Stations 30.
A "subscriber" is defined herein as a system user. FIG. 1 shows "Subscriber Unit" 26 (SU) which is an individual communication terminal which communicates directly with a satellite 12 via a radio link. SUs 26 may be hand-held, portable cellular telephones adapted to transmit subscriber data to and receive subscriber data from satellites 12. "Subscriber data" is defined herein as data (e.g., voice, paging, or fax data) originating from or terminating at a SU 26.
A "Ground Communication Station" (GCS) is defined herein as a terrestrial communication facility capable of interfacing ground based equipment (e.g., Gateway 22 or System Control Segment 28) with satellites 12. FIG. 1 shows Gateway GCS 30 (GW-GCS) associated with Gateway 22 and System Control Segment GCS 24 (SCS-GCS) associated with System Control Segment 28. SCS-GCSs 24 perform data transfer and telemetry, tracking, and control functions for the constellation of satellites 12. GW-GCSs 30 perform data transfer between satellites 12 and Gateways 22.
A "Gateway" 22 (GW) is defined herein as an equipment facility, typically ground-based, which is capable of interfacing GW-GCS 30 (and thus satellites 12) with ground-based equipment such as, for example, a public switched telephone network (PSTN), not shown. GWs 22 may perform call processing functions in conjunction with terrestrial telephony equipment (e.g., PSTN equipment) and satellites 12. GWs 22 communicate with the rest of communication system 10 via GW-GCSs 30. GWs 22 need not be co-located with GW-GCSs 30. GWs 22 are preferably coupled to GW-GCSs 30 via land-lines, although this is not essential. In an alternate embodiment, GWs 22 may be coupled to GW-GCSs 30 via fiber optic links, radio links or other transmission mediums.
A "System Control Segment" 28 (SCS) is defined herein as a control facility, typically ground-based, which controls operation of communication system 10. SCS 28 communicates with the rest of communication system 10 via SCS-GCS 24. SCS 28 need not be co-located with SCS-GCS 24. SCS 28 is preferably coupled to SCS-GCS 24 via land-lines, although this is not essential. In an alternate embodiment, SCS 28 may be coupled to SCS-GCS 24 via fiber optic links, radio links or other transmission mediums.
A "Ground Terminal" (GT) is defined herein as any communication facility (e.g., GW-GCS 30, SCS-GCS 24, SU 26, etc.), located on or near the surface of a celestial body (e.g., earth), which is capable of communicating directly with a satellite 12. Under this definition, SCS 28 and GW 22 are not GTs.
A "system node" is defined herein as a satellite 12, SU 26, GW 22, SCS 28, SCS-GCS 24 or GW-GCS 30. Only one each of GW 22, SCS 28, SU 26, SCS-GCS 24 and GW-GCS 30 is shown in FIG. 1 for clarity and ease of understanding. Those of skill in the art will understand based on the description herein that additional system nodes may be desirable, depending upon the needs of the communication system.
A "channel" is defined herein as a communication access opportunity (e.g., a Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), or Code Division Multiple Access (CDMA) slot). A "link" is defined herein as a communication channel established between one system node and another system node, independent of what kind of communication access protocol is used. "System data" is defined herein as data necessary for control and operation of the system 10 (e.g., system control information, call setup data, signalling data etc.).
FIG. 2 illustrates communication links between several system nodes in communication system 10 (FIG. 1). SUs 26 exchange subscriber data with satellites 12 over radio links referred to hereinafter as "subscriber links" 40. Subscriber links 40 are generally low bandwidth channels that carry subscriber data. As used herein, the term "antenna" is intended to refer to any device for transmitting and/or receiving electromagnetic energy. Subscriber signals are desirably "brought up" into satellite 12 via dedicated, cellular subscriber antennas 42 on board satellite 12 which projects onto the surface of the celestial body a pattern of "cells" 44 within which SU 26 must be located in order for communications to occur between SU 26 and subscriber antennas 42. Each cell 44 is assigned sets of subscriber channels which will not interfere with subscriber channels of surrounding cells. In the preferred embodiment, each subscriber channel may be multiplexed (e.g., TDMA, FDMA, CDMA, etc., or a combination thereof) to allow multiple users to communicate using a single subscriber channel. In an alternate embodiment, the subscriber channels may not be multiplexed.
An "up/down link" is defined herein as a radio channel between a ground based communication facility (e.g., SCS-GCS 24 or GW-GCS 30) and a satellite 12. Satellites 12 exchange data with GW-GCSs 30 and SCS-GCSs 24 via two types of up/down links: "feeder links" 46 and "control links" 48. Feeder links 46 between satellites 12 and GW-GCSs 30 carry system data and subscriber data. Control links 48 between satellites 12 and SCS-GCSs 24 generally carry only system data. Feeder 46 and control 48 links desirably use dedicated satellite down-link antennas 50, GW-GCS up-link antennas 52 at GW-GCS 30 and SCS-GCS up-link antennas 54 at SCS-GCS 24.
An "up-link" is defined herein as a link from a GW-GCS 30 or SCS-GCS 24 to a satellite 12. A "down-link" is defined herein as a link from a satellite 12 to a GW-GCS 30 or SCS-GCS 24.
A "cross-link" is defined herein as a radio channel between one orbiting satellite and an orbiting neighbor satellite. "Neighbor satellites" are defined herein as satellites 12 that are capable of establishing direct communications with each other. "Non-neighbor satellites" are defined herein as satellites 12 that are not capable of establishing direct communications with each other (e.g., due to range limitations, interference restrictions, or geographical barriers). Satellites 12 exchange data with neighbor satellites 12 via cross-links 56, although satellites 12 may also exchange data with non-neighbor satellites. Cross-links 56 carry system data and subscriber data. Subscriber data from SUs 26 are converted into cross-link signals within satellites 12 when the subscriber data must be communicated to another satellite 12 or to GW-GCS 30. Cross-link antennas 58 on board satellites 12 are desirably used to transmit and receive signals directly from other satellites 12. If simultaneous communication is desired with multiple satellites 12, multiple cross-link antennas 58 may be used. Referring to FIG. 1, satellite 12 communicates "fore" with another satellite 12 leading it within the same orbit plane and communicates "aft" with another satellite 12 following it. Also as herein defined, satellite 12 communicates "left" or "right" with other satellites 12 in an adjacent orbit plane, depending on which adjacent plane the other satellite 12 is located in. "Left", "right", "fore", and "aft" are defined as if the observer was riding on the satellite 12 facing in the direction of the satellite motion.
In a classical "bent pipe" communication system of the prior art, satellites 12 do not communicate directly with other satellites 12 over cross-links 56 (FIG. 2). Instead, signals from one satellite 12 are sent on relay links 60 to relay 62. Relay 62 may then communicate the data in the signals to land-based radio or land-line equipment, or transmit it up to another satellite 12 or to another node on relay ground-links 64. Relay 62 may be earth-based, airborne, or space-based as long as it is within sight of the transceiving satellites. Another feature of the typical "bent pipe" system is that the satellites 12 do little more than receive signals on one frequency and transmit them on another, independent of the origin or destination of the signals, i.e., no information processing takes place in the "bent pipe" satellite transponder.
For successful operation of communication system 10, management of each system node's finite resources is necessary to maximize system efficiency. For example, communication system 10 should not allow a busy metropolitan area (e.g., Tokyo) to consume all the stored energy on board a satellite that will be needed, say, thirty minutes later to support another busy metropolitan area (e.g., New York City). Stored energy on-board a satellite 12 is typically limited by the ability of the satellite's solar collectors to convert solar flux into electrical energy, and by the ability of the satellite's batteries to store the converted solar energy. Thus, the state of charge or amount of energy stored in the satellite's batteries is an important physical constraint which must be managed.
Satellite antennas 42, 50, 58 are also limited resources. Because a finite number of antennas exist on-board a satellite 12, the number of ground terminals 26, 24, 30, 62 and other satellites 12 that may communicate to or through the satellite 12 is limited by the number of antennas 42, 50, 58. For a cellular satellite communication system, the number of subscriber cells 44 per antenna 42 and the number of subscriber channels per cell define additional resource limitations.
In addition to physical resources of a space-based communication system, operating restrictions also affect resource usage. Operating restrictions may be interference, licensing and spectrum restrictions. For example, a satellite 12 may be required to turn off some of its cells 44 while passing over a particular location on the earth to avoid interfering with sensitive ground-based equipment (e.g., radio-astronomy receivers).
Resource management is critical in a space-based communication system because the resources are severely limited and are often not easy to increase or replenish due to the remote nature of the satellites 12.
Prior methods of resource management of a satellite system entail manual creation of rule sets that are turned into decision trees defining control room procedures for humans to follow when particular events or states occur within the system.
Because the rule sets and decision trees are manually created, and operation of the system requires humans to collect system information and react to it in accordance with the control room procedures, a large staff is typically needed to operate a satellite system.
One disadvantage to using human staffs is increased cost of operating a system. In general, a more cost effective solution to system management is to employ automated processes rather than humans.
Another disadvantage to using humans in the decision making process is that response times to system events are slow. Slow response times may have detrimental effects. For example, if a satellite reports that it (or another remotely located node) is running dangerously low on stored energy, a human operator must receive the information and determine what course of action to take to alleviate the problem (including analyzing the effects on the rest of the system), gain the required approval to take such action, and implement the change. Meanwhile, if the satellite runs too low on stored energy, its on-board sub-systems may drop off line and the whole satellite may enter a survival mode or become inoperative, during which time the affected satellite is unable to support subscriber traffic. In a single coverage system (i.e., many points on the surface of the earth are serviced by only one satellite at a time), this is particularly critical. While the affected satellite is off line, all traffic being supported by that satellite is dropped. The affected satellite will cause all subscriber calls and control and feeder links to terminate everywhere in its path. This represents service degradation and damage to the reputation of the system. If the satellite is permanently taken off line, weeks of delay may result while the satellite is replaced.
Because prior art resource management is performed predominantly by humans, the limits of human efficiency set boundaries on the efficiency of resource management, and the size and complexity of satellite systems which can be effectively controlled. The problem worsens as the complexity of a satellite system increases because the amount of dynamic system resources also increases.
With rapidly advancing communications technologies, larger and more complex space-based communication systems are desirable. Because a large and complex space-based communication system cannot be efficiently managed using predominantly human staffs, an adaptive network resource management function is desired which allows rapid and accurate management of a communication system's physical resources. The ability to rapidly replan resource usage when unanticipated emergency or out-of-tolerance situations occur minimizes system down-time and maximizes system efficiency and customer satisfaction.
Therefore, computationally efficient methods are needed to rapidly manage resources of a satellite cellular communication system.
Further, in order for the system to be able to respond to service requirement changes in real time, it is desirable that resource analysis and management be as automated as is possible. Automated management is especially desirable to allow rapid planning of limited resource usage of a constellation of communications satellites and associated ground stations.